ppc/kvm: Skip writing DPDES back when in run time state

ppc/kvm: Skip writing DPDES back when in run time state

On POWER8 systems the Directed Privileged Door-bell Exception State
register (DPDES) stores doorbell pending status, one bit per a thread
of a core, set by "msgsndp" instruction. The register is shared among
threads of the same core and KVM on POWER9 emulates it in a similar way
(POWER9 does not have DPDES).

DPDES is shared but QEMU assumes all SPRs are per thread so the only safe
way to write DPDES back to VCPU before running a guest is doing so
while all threads are pulled out of the guest so DPDES cannot change.
There is only one situation when this condition is met: incoming migration
when all threads are stopped. Otherwise any QEMU HMP/QMP command causing
kvm_arch_put_registers() (for example printing registers or dumping memory)
can clobber DPDES in a race with other vcpu threads.

This changes DPDES handling so it is not written to KVM at runtime.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Message-Id: <20190923084110.34643-1-aik@ozlabs.ru>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>